At present, the cluster communication system is developed to meet the requirements of industry users, such as command and dispatch and so on, and the cluster communication system is the private wireless communication system oriented specific industry applications. The main characteristics of the cluster communication system which is different from the public radio mobile communication system is that, in addition to provide the two-direction call function of the mobile phone, the cluster communication system can provide the cluster (group) call and the whole call within the system, and even establish the call priority level, and it can also perform communication functions which the normal mobile phones do not have in the system, such as priority level call, emergency call and other communication; it can also provide special functions, such as dynamic reassembly, the virtual special network within the system and so on. These features are particularly suitable for dedicated communications for police, national safety departments and command and dispatch requirements for airport, customs, public transport, disaster relief and so on, so the cluster communication system was widely applied in government departments, public safety, emergency communications, electricity, civil aviation, petrochemical, military and enterprises and other fields, and it is a multi-purpose, high-performance wireless communication system, and it formed a dedicated communication network which is independent of the public mobile communication network all over the world.
With the growing shortage of spectrum resources, the voice for the public network cluster is more and more high. Public network cluster refers to developing specific cluster communication services based on the existing public mobile communication network.
To ensure the capability that the cluster user equipment can keep calling in the case of critical services, the 3GPP has proposed the requirement for the pattern of IOPS (Isolated E-UTRAN Operation for Public Safety). The pattern of IOPS refers to (N)eNB, i.e., (nomadic) evolved Node B, (or a group of (N)eNBs) can provide a certain level of communication capability for the cluster user in the case where the circuit is interrupted or the circuit fails to lead that the signaling and data cannot be transmitted normally.
Since the IOPS scenario includes one or several connected (N)eNBs and there is no Backhaul Link or a bandwidth-limited backhaul link in the IOPS scenario, it means that the coverage range of IOPS is very limited, and once the user leaves its coverage area, communication interruption must be cased, especially when the user is the group call sponsor or dispatch station, the entire group of communications will be interrupted, which cannot be tolerated by the cluster communication. To avoid this situation, an early warning mechanism is required to be taken to make the user return to the effective communication area to ensure that the communication of the user will not be interrupted.